1. Field of the Invention
The present invention relates to a recording apparatus and a recording method, more specifically, recording an image using a recording head for discharging ink.
2. Description of the Related Art
An inkjet recording system printer records an image on a recording medium by scanning a recording head, which discharges ink based on recording data of the image, in a direction (scanning direction) which is orthogonal to a conveyance direction (sub-scanning direction) of the recording medium, such as a sheet of paper. Among inkjet recording systems, a bubble jet recording system is a system which discharges ink from a discharge port by the pressure of a bubble generated by heating the ink rapidly to turn it into a gas by an electrothermal transducer (hereinafter, sometimes referred to as a “heater”).
FIGS. 14A and 14B illustrate a prior art recording head 11 of a bubble jet recording system. FIG. 14A is a plan view of the main parts of the recording head 11, and FIG. 14B is a cross-sectional view taken along the line 14B-14B of 14A.
In FIGS. 14A and 14B, a substrate 1, heaters 2, an orifice plate 4, and discharge ports 5 are provided. Further, ink flow paths 10 are formed between the substrate 1 and the orifice plate 4, and a partition wall 9 is provided between the plurality of ink flow paths 10. In addition, an ink supply port 8 is formed on the substrate 1. The heaters 2 are provided on the substrate 1 so as to face the discharge ports 5. A protective film is formed on the surface of the heaters 2. Ink is supplied from a common fluid chamber (not-illustrated) in communication with the ink flow paths 10 to the respective discharge ports 5 via the ink flow paths 10. When a drive pulse is applied to the heaters 2 provided at a position that faces the respective discharge ports 5, the ink is rapidly heated, and thereby ink is discharged from the discharge ports 5.
However, even when a similar drive pulse is applied to the heaters 2, the ink discharge amount differs depending on the temperature of the ink near the heaters 2. Further, if drive pulses are continuously applied to the heaters 2, heat accumulates in the recording head 11. Thus, if ink is continuously discharged from the recording head 11, this causes an increase in the ink temperature near the heaters 2, and thereby the ink discharge amount increases.
FIGS. 15A to 15C are prior art diagrams illustrating the relationship between the recording head temperature and the ink discharge amount. Here, since measuring the ink temperature near the heaters 2 is practically difficult, generally the temperature is measured by providing a thermistor in the recording head, which acts as a substitute measuring for the ink temperature near the heaters 2. FIGS. 15B and 15C respectively illustrate the relationship between the recording head temperature Th and the recording position, and between the ink discharge amount Vd and the recording position when the recording head 11 is scanned from a recording position A to a recording position B while the ink is continuously discharged from the recording head 11 as illustrated in FIG. 15A. As illustrated in FIGS. 15B and 15C, the recording head temperature Th increases as the scanning proceeds from the recording position A to the recording position B, and with the increase of this recording head temperature Th, the ink discharge amount Vd also increases. Therefore, as illustrated in FIG. 15A, the recorded image has recording density increasing from recording position A towards recording position B, so that density unevenness occurs along the scanning direction.
Accordingly, in the recording method discussed in Japanese Patent Application Laid-Open No. 8-156258, valid data from among the recording data is counted for each scan, and when this count value exceeds a reference value, the subsequent recording data in that scan is thinned, or the pulse width of the pulse signals driving the recording head 11 is reduced. According to this recording method, the effects of an increase in the ink discharge amount caused by the increase in the recording head temperature, namely the density unevenness in the scanning direction, can be reduced.
However, in the recording method discussed in Japanese Patent Application Laid-Open No. 8-156258, in some cases, regions where the recording density suddenly changes exist, which causes image quality to deteriorate.
This phenomenon will now be described referring to FIG. 16. FIG. 16 is a prior art diagram illustrating a region where the recording density suddenly changes, which is formed on a recording medium 21 when an image is recorded by the recording method discussed in Japanese Patent Application Laid-Open No. 8-156258. In FIG. 16, an image is recorded by scanning the recording head 11 in a scanning direction indicated by arrow A while ink is discharged from the recording head 11, and by conveying the recording medium 21 in a sub-scanning direction indicated by arrow B between scans. That is, among the regions to be recorded in the recording medium, an image is recorded on a recording region 22 of the recording medium 21 by successively recording the image on scanning regions (bands) 23, over each of which the recording head 11 performs one scan. Here, the recording head 11 records on the recording medium 21 by discharging ink only when scanning in the same direction as the arrow A.
In the recording method discussed in Japanese Patent Application Laid-Open No. 8-156258, recording is performed by counting valid data from among the recording data for each scan, and when the count value exceeds a reference value, thinning the subsequent recording data in that scan, or reducing the pulse width of the pulse signals, which drives the recording head 11. Cases, when the count value exceeds the reference value and thinning the recording data is performed, will be described below.
At a position of a boundary line 24 illustrated in an N-th region to be recorded by an N-th scan of the recording head 11 (N-th scanning region), the count value of valid data at this position reaches the reference value. This boundary line 24 is an imaginary line illustrating that the recording data was thinned from this position.
In the region 25A, which is located in the left side of the boundary line 24, since the temperature of the recording head has increased caused by the scanning of the recording head 11, the ink discharge amount in this region is increased. Therefore, in the region 25A, the image is recorded with a higher recording density.
However, the change in recording density caused by the increase in the ink discharge amount in the region 25B, which is located on the right side of the boundary line 24, can be suppressed by thinning the recording data. By thinning the recording data in the midst of scanning in this way, density unevenness in the scanning direction can be mitigated even if the ink discharge amount increases due to an increase in the recording head temperature.
However, if the recording data is thinned from a certain position such as the boundary line 24, the recording density temporarily decreases across the boundary line 24. As a result, a substantial difference in recording density occurs between the position immediately after the start of thinning of the recording data in the right side of the boundary line 24 and the left side of the boundary line 24 where the recording data is not thinned. That is, the vicinity of the boundary line 24 becomes a region where recording density suddenly changes. This is visually perceived as density unevenness, thus causing deterioration in image quality.
Further, it is assumed that also in the next scanning region of the N-th scanning region (N+1-th scanning region), the count value of the valid data reaches the reference value at the similar position in the scanning direction as the N-th scanning region. In such a case, because regions where the recording density suddenly changes are continuous in the sub-scanning direction, this is more easily perceived as density unevenness, which causes a further deterioration in image quality. Thus, if regions where the recording density suddenly changes are continuous in the sub-scanning direction, that will deteriorate the quality of the image to be recorded on the recording medium to a large extent.